1. Field of the Invention
The present invention relates to a cryopump.
2. Description of the Related Art
A cryopump is a vacuum pump trapping gases on a cryogenically-cooled cryopanel by means of condensation or adsorption. The cryopump pumps gases from a vacuum chamber on which the cryopump is mounted.
The cryopump normally includes a first cryopanel cooled at a certain temperature and a second cryopanel cooled at a lower temperature than the certain temperature. The first cryopanel includes a radiation shield. Along with use of the cryopump, a condensing layer of gases grows on the second cryopanel. The condensing layer can eventually contact the radiation shield or a part of the first cryopanel. In this case, gases vaporize again at a contacting part, and pressure in the cryopump increases. Since then, the cryopump cannot play an actual role of pumping of the vacuum chamber sufficiently.
Thus, the total amount of gas condensed at the time when the condensing layer contacts the first cryopanel provides a gas capacity limit of the cryopump.